The present invention relates to a nitride semiconductor light-emitting element used in optical devices such as light-emitting diodes, semiconductor laser diodes, etc., and a process for producing said element.
Nitride semiconductors containing Group III elements such as Al, Ga and In and a Group V element such as N and represented by AlGaInN have come to be often used in these years as semiconductor materials for visible-light-emitting devices and high-temperature working electron devices. Particularly, their practical application in the field of blue and green light-emitting diodes and their development in the field of bluish-purple laser diodes are now in progress.
For producing light-emitting elements by using such nitride semiconductors, a process of allowing nitride semiconductor thin-film crystals to grow by a metal organic chemical vapor deposition method has recently been a leading process. This process comprises feeding gaseous sources of Group III elements such as metal organic compound gases, e.g., trimethylgallium (hereinafter abbreviated as TMG), trimethylaluminum (hereinafter abbreviated as TMA) and trimethylindium (hereinafter abbreviated as TMI), and a gaseous source of Group V element such as ammonia, hydrazine or the like, into a reaction tube in which is placed a substrate made of, for example, sapphire, SiC and GaN, and maintaining the substrate at elevated temperatures of approximately 700xc2x0 C.-1100xc2x0 C. to allow an n-type layer, a light-emitting layer and a p-type layer to grow to make a laminate on the substrate. The n-type layer is allowed to grow while feeding monosilane (SiH4), germane (GeH4) or the like as gaseous source of n-type impurity elements together with the gaseous sources of Group III elements. The p-type layer is allowed to grow while feeding cyclopentadienylmagnesium (Cp2Mg), dimethylzinc (Zn(CH3)2) or the like as gaseous source of p-type impurity elements together with the gaseous source of Group III element.
After the growth and formation of the nitride semiconductor thin-film crystals on the substrate, an n-side electrode and a p-side electrode are formed on the n-type layer surface and the p-type layer surface, respectively, for example, by a metal vapor deposition method. The product thus obtained is separated into chips in the step of dicing to obtain light-emitting elements.
In nitride semiconductors, p-type conduction has been difficult to attain but has recently become controllable by an electron beam irradiation method or an annealing method. Thus, a nitride semiconductor doped with a p-type impurity element is allowed to grow, and then, the p-type impurity element is activated to function as an acceptor by breaking the bond between hydrogen and the p-type impurity element (the p-type impurity element has been inactivated by the bonding with the hydrogen) by electron beam irradiation method or annealing method and releasing the hydrogen from the nitride semiconductor. As the result, the nitride semiconductor can be made into a p-type semiconductor. In particular, the annealing method is considered to be a practical production method, because it requires a simple equipment therefor and a short time for making the nitride semiconductor into a p-type semiconductor, as well as the nitride semiconductor can be made into a p-type semiconductor uniformly also in the direction of depth of the nitride semiconductor.
However, in the annealing method, impurities such as oxygen and carbon tend to intrude into the nitride semiconductor film through the nitride semiconductor surface during heat treatment, so that the impurities are present particularly in the vicinity of the surface. Said method, in which hydrogen is expelled from the thin film crystal by breaking the bond between the p-type impurity and hydrogen, cannot achieve complete hydrogen expelling from the crystal, so that the presence of hydrogen preferentially in the vicinity of the nitride semiconductor surface is unavoidable. Thus, there has been the following problem: an electrode formed on a p-type nitride semiconductor containing hydrogen, oxygen, carbon and the like preferentially in the vicinity of the surface does not exhibit satisfactory ohmic characteristics.
For solving this problem, for example, JP-A-10-135575 has proposed a method comprising suppressing the formation of a bond between a p-type impurity and hydrogen by reducing the amount of hydrogen contained as a carrier gas in an atmosphere gas in a reaction tube, during the metal organic chemical vapor deposition of a nitride semiconductor doped with the p-type impurity, and attaining p-type conduction without any treatment after the deposition. Appl. Phys. Lett. Vol. 72, No. 14, p. 1748 (1998) contains detailed experimental results obtained by the study group which show that satisfactory p-type conduction is attained at a hydrogen content of 2.4% or less. According to this report, inactivation by hydrogen occurs at a hydrogen content of 3.7% or less.
However, when the nitride semiconductor is allowed to grow at a low hydrogen content of the carrier gas as proposed by the prior art above, the migration of atoms on the semiconductor crystal surface is suppressed, so that the crystallinity is more easily deteriorated at the low hydrogen content than at a high hydrogen content. Therefore, when the whole of p-type layer is uniformly formed at a low hydrogen content in the carrier gas, the crystallinity is deteriorated particularly in the upper portion of the p-type layer. Consequently, a satisfactory ohmic contact is difficult to obtain between the p-type layer and the p-side electrode formed on the outermost surface of the p-type layer, resulting in undesirable influences on the light-emitting characteristics of the resultant light-emitting element.
An object of the present invention is to provide a nitride semiconductor light-emitting element which is synthetically designed to possess desirable harmony between the p-type electric properties and crystallinity in the p-type layer, is excellent in light-emitting properties, and requires only a low operating voltage owing to the satisfactory ohmic contact.
The present inventor earnestly investigated the structure of p-type layer in nitride semiconductor light-emitting elements. As a result, he found that formation of the p-type layer out of at least two plies, the first p-type sublayer in contact with a light-emitting layer and the second p-type sublayer provided so as to farther distanced from the light-emitting layer than the first p-type sublayer, by allowing the first p-type sublayer to grow in an atmosphere gas having a low hydrogen content and allowing the second p-type sublayer to grow in an atmosphere gas having a hydrogen content higher than that used for allowing the first p-type sublayer to grow, achieves, in the first p-type sublayer, high activation rate of the p-type impurity which keeps the hole content high and, in the second p-type sublayer, restoration of the deteriorated crystallinity of the first p-type sublayer.
By the above features it becomes possible to obtain a light-emitting element excellent in both p-type electric properties and crystallinity in the p-type layer.
He also found a process for producing a nitride semiconductor light-emitting element comprising an n-type layer, a light-emitting layer and a p-type layer accumulated in this order, said p-type layer being a laminate comprising at least two p-type sublayers, the first p-type sublayer in contact with a light-emitting layer and the second p-type sublayer provided so as to farther distanced from the light-emitting layer than the first p-type sublayer, wherein at least the p-type layer is allowed to grow by a metal organic chemical vapor deposition method, which process comprises the steps of (a) allowing the first p-type sublayer to grow by a metal organic chemical vapor deposition method in a reaction tube and (b) allowing the second p-type sublayer to grow by a metal organic chemical vapor deposition method in the reaction tube, the hydrogen content in the total mixture of gases fed into the reaction tube in step (a) being lower than the hydrogen content in the total mixture of gases fed into the reaction tube in step (b), leading to, in the first p-type sublayer, high activation rate of the p-type impurity which keeps the hole content high and, in the second p-type sublayer, restoration of the deteriorated crystallinity of the first p-type sublayer.
When such a production process is adopted, it becomes possible to produce a light-emitting element excellent in both p-type electric characteristics and crystallinity in the p-type layer.